Electric furnace



Feb. 14, 1939. c. M. WEINHEIMER ET AL. 2,147,071

ELECTRIC FURNACE Filed July 27,

' 0a r ded.

/7 Mew fer/mew Z" Patented Feb. 14, 1939 UNITED STATES PATENT OFFICE ELECTRIC FURNACE Application July 27, 1936, Serial No. 92,789

9 Claims.

The present invention is in general concerned with improvements in an electric furnace, and more particularly has to do with the construction of the furnace electrode and terminals therefor.

In electric furnaces of the type utilizing electrodes in the form of a graphite or carbon resistor heating element in the melting chamber, diiliculties have been experienced due to the rapid disintegration or deterioration of the element at even moderate temperatures, when in contact with oxygen.

Attempts have been made to overcome this difficulty by providing rather complex arrangements for protecting the resistor. In some cases,

attempts have been made to prevent disintegration of the electrode by operating the resistor in an atmosphere of producer gas, by introducing the gas into the melting chamber. This proved to be impracticable as it is necessary to frequently open the furnace for charging and pouring. Moreover, any leakage of air into the furnace will produce an explosive mixture, whereby life and property become imperiled.

In other cases, the resistor is surrounded with a casing of silica, through which an inert gas is forced, and at high temperatures the silica combines with the carbon of the electrode to form a protective shield of silicon-carbide.

The present invention proposes to protect the resistor by constructing the electrode of a porous graphitic material in the shape of a tube. An inert or reducing gas is introduced into this tube at sufficient pressure to force the gas to seep 5 through the wall of the resistor and thus protect the particles of material composing the resistor from disintegration and deterioration when in an atmosphere containing oxygen.

The invention further seeks to improve and 49 protect the electrode terminals. In the prior art constructions, attempts have been made to provide 1 R terminal heating to act as a counterflow to heat conducted from the melting chamber. Terminals of this type are relatively ineillcient and 15 due to the PR losses, reduce the amount of cur rent available for actually heating the metal in the melting chamber.

The present invention seeks to provide a construction in which the electrical resistance is kept 50 low and by a proper selection of materials conduction of heat outwardly through the terminal is prevented and oxidation reduced to a minimum, whereby greater input current may be utilized for actually heating the metal in the melt- 65 ing chamber.

A further object is to provide a stationary terminal in a rotatably mounted melting chamber, this terminal being so constructed as to enable the electrode to be easily and conveniently changed. 6

It is also an object of the invention to provide an improved terminal construction having a relatively high electrical conductivity and which will at the same time act as a barrier to the flow of heat therethrough. 10

A further object is to provide novel means for v centering and supporting a stationary electrode in a rotatable electric furnace.

Still another object of the invention is to provide an electric furnace having the electrode and 15 electrode terminals so constructed as to enable the carrying out of our improved method of protecting the electrode and terminals from deterioration due to oxidation.

Other objects and features of the invention will 20 more fully appear from the following detailed description taken in connection with the accompanying drawing which illustrates a single embodiment thereof, and in which:

Figure 1 is an elevational view of an electric 25 furnace mounted for rotative movement, and having cutaway portions with sections taken through the terminals and end walls of the melting chamber to show their cooperative relationship;

Figure 2 is a transverse sectional view taken through one of the electrode terminals, taken substantially on line II-II of Figure 1; and

Figure 3 is an enlarged transverse section through the electrode, taken substantially on 5 line III-III of Figure 1.

As shown in the drawing:

In the illustrated embodiment of the herein described invention, there is disclosed in Figure 1 an electric furnace which embodies a shell structure In defining a melting chamber. The shell at each end is provided with circumferentially extending rails II and H which cooperate in each case with supporting rollers 13 so that the shell, which in this case is cylindrical, may be rotated about its longitudinal axis. Any suitable driving means may be employed for rotating the shell, and if desired one end of the shell may be provided with a circumferentially extending gear, as shown at M, which may be connected thru a suitable pinion drive to power means such as an electric motor or the like, not shown.

The ends at the furnace shell are formed by east end plates i5 and Hi. 'The interior of the furnace shell is suitably lined with protective material such as fire brick or the like and at the ends of the shell, extending inwardly from the end plates, a plurality of layers of fire bricks or the like as shown at I! and i8 are provided. These layers, it will be noted, are of less thickness adjacent the metal end plates than the layers which are spaced from the end plates. Covering the innermost layer of thermal insulation, is a layer of refractory material E9. The end plate It, the layers of thermal insulation H and i8, and the refractory material l9 are centrally provided with apertures which are aligned and form an opening for receiving a cylindrical sleeve 20.

.The sleeve 20 is constructed of a refractory material such as aluminum oxide, this material being commercially known as alundum.

The melting chamber contains an electrode 2i which is constructed of a graphitic material. This electrode forms a resistor to the flow of electric current, and will become heated to a high temperature for reducing the metal in the melting chamber. The electrode is provided with a longitudinally extending central bore 22 and is threaded at either end at 23 and 24 for threadedly engaging with terminal end blocks 25 and 26 which are of larger diameter than the electrode proper and are likewise constructed of graphite. The terminal blocks at each end of the electrode are supported for relative movement in the associated sleeves 20-20 of refractory material. Each terminal block is of sufficient length to project to the exterior of the furnace shell and forms conducting means for carrying the current from suitable current supply conductors disposed outside of the furnace shell.

During the melting of the metal in the melting chamber, oxygen will be present. Unless some means is provided for protecting the electrode, this oxygen will, at the high temperature of the melting chamber, cause a disintegration or deterioration of the electrode, whereby its diameter would soon decrease to such an extent as to seriously impair its usefulness.

In order to overcome this condition, it is proposed to conduct an inert or reducing gas to the interior of the electrode, this gas being at such pressure as to cause the gas to seep out through the pores of. the material composing the electrode and thus protect the material against disintegration and deterioration in the oxygen atmosphere.

For thispurpose, one gas which has been found to be very suitable comprises a mixture oi a high percentage of nitrogen and a low proporti n of hvdrogen. More specifically, a gas containing 93% of nitrogen and 7% of hydrogen has been successfully used for this purpose. Using an electrode of graphite having a porosity of approximately it has been found that a pressure of approximately 5 pounds will be required to cause seepage of the gas through the pores in the wall of the electrode. While the above proportions of the gas constituents have been given and the pressure of gas necessary for a' porosity of 30%, it will be appreciated that other gases than the one specifically described may be used and that the gas pressure may be whatever is necessary in order to afford seepage of the gas through the electrode material, this pressure of course depending upon the porosity of the material.

The end of terminal block 26, which is connected to the associated end of the electrode, is provided with an end socket 21 which is in alignment with and forms a continuation of the tubular bore of the electrode. This socket extends V to a depth which is substantially coincident with the cast end plates of the furnace shell and the bottom of the socket is at the outermost end of the sleeve 20 so that during the operation of the furnace, gas will also seep out through this end of the terminal block and protect the end of the block from deterioration the same as in the case of the electrode.

Terminal block 25 is similarly constructed to block 26, except that in this case instead of having an end socket the terminalblock is provided with a longitudinal bore 28 which extends the full length of the block and is arranged at its outer end for connection to a gas supply pipe 29. The protecting gas will seep through the wall of block 25 in a similar manner to the seepage of gas in the end of block 26, and protect the terminal block against deterioration.

Surrounding the terminal blocks and abutting the outer end of the sleeves 202G, ring shaped plates 30'30 are secured to the end plates of the furnace shell. The plate 30 is of thermal insulating material which also acts as an electrical insulator. For this purpose, a composition of cement and asbestos commercially known as Transite, has been found to be very suitable. The outer surface of this plate is provided with an annular groove 3| which is adapted to receive a peripherally extending end flange S2 at the inner end of a contact ring 33.

The contact ring 33 is constructed of a metal having relatively high electrical conductivity and relatively low thermal conductivity. For this purpose, a metal containing a combination of nickel and chromium, commercially known as Hoskins Cast Nickel-Chromium Alloy, has been found to be very suitable. While the composition of this alloy may vary somewhat for different,

conditions of use, in general it comprises substantially eighty per cent. nickel and twenty per cent. chromium. The contact 33 is diametrically split and also has diametrically opposed slots 3 which cooperate with the spaced ends defined by the diamctric split to divide the contact into seg mental sections. Since these separations extend transversely of the contacts, circulating currents will be prevented therein.

Surrounding the contact 33, there is concen trically disposed a diametrically split ring 35 of carbon. This ring is of relatively thin construetion, but is of sufiicient width to present a large surface contact with the nickel chromium contact 33. The carbon contact due to its large area presents very little resistance to the flow of cur rent therethrough, but due to its low thermal conductivity, this contact serves as a barrier to prevent heat conduction outwardly from the graphite terminal block.

Current is supplied to the terminal through a rigidly supported bus, which may be of copper, aluminum or other material having high elec trical conductivity. In this case, the bus is composed of two bars 36 and 37 having oppositely curved semi-circular end portions which fit around and engage the outer surface of the carbon' contact 35. The terminal assembly is clamped into tight surface engagement by means of a pair of semi-circular clamping plates 38 and 39 which are provided along their cooperative edges with clamping bolts 40 and 4!, the latter extending not only through the clamping members but also through the bus bar as well.

For thermally insulating the outer ends of the graphite terminal block, the nickel-chromium contact and the carbon contact, a disc 42 of thermal insulation is disposed in abutting relation. The circumferential edge of this disc is coincident with the outer surface of the carbon contact and this disc is clampingly engaged by the bus bars so as to be held in position.

As before stated, the bus bar is supported in any desired manner, and it will be apparent that as the furnace shell rotates, the bus bar retains the terminal and elect-rode against rotation therewith. Moreover, the electrode is held centered at the axis of revolution of the furnace shell by means of the above described flange and groove connections between the nickel chromium contacts and the thermal insulating plates 30.

The bus bars, when connected to the novel terminal construction described herein, also act to prevent longitudinal movement and removal of the electrode from the melting chamber. However, the terminal is of such construction that it may be easily disassembled so as to permit convenient and easy removal of the electrode when desired.

From the foregoing description, it will be apparent that the herein-described invention provides a novel electric furnace having a stationary electrode and electrode terminals which are protected in a novel manner from disintegration and deterioration; novel means for maintaining the electrode and terminal parts in operating position; terminal connections which have a high degree of electrical conductivity but a comparatively low degree of thermal conductivity whereby greater input current may be utilized for actually h ating the metal to be melted; and a novel arrangement of electrode and electrode terminal parts which enables the carrying out of the herein described novel method of protecting the electrode from deterioration and disintegration.

Although we have described and illustrated a preferred form of the invention in detail, it is of course to be understood that the invention is not to be thus limited but only insofar as defined by the scope and spirit of the appended claims.

We claim as our invention:

1. An electric furnace comprising a melting chamber having a refractory lining, a resistor in said chamber, a terminal block of graphitic material in threaded engagement with an end of said resistor and extending through said lining to the exterior of the chamber, a sleeve of aluminumoxide disposed between said block and lining, a ring of thermal insulating material disposed at the outer end of said sleeve around said block and making sliding contact therewith, a nickelchromium alloy member composed of substantially eighty per cent. nickel and twenty per cent. chromium disposed around the projecting end of said block, a carbon contact member in engagement with and substantially surrounding the nickel-chromium member, a bus bar eleotrically contacting the carbon member, and means for clamping the bus bar, carbon contact member, nickel-chromium member and projecting end of the block into tight engagement.

2. An electrical furnace comprising a melting chamber, a resistor electrode in said chamber, a terminal block of relatively low heat conductivity projecting through a wall of said chamber, one end of said block being electrically connected to said electrode and its other end disposed outside said chamber, a metallic sleeve around the projecting end of said block, said sleeve being composed of a material having relatively high electrical conductivity and relatively low thermal conductivity, a current supply conductor around sa d sleeve. and a heat barrier disposed between said sleeve and current supply conductor, said barrier being composed of a material having relatively low thermal capacity and of such dimensions as to have low resistance to the flow of current.

3. In an electric furnace a melting chamber supported for rotative movement, a stationary electrode in said furnace, an electrode end terminal block projecting through the chamber wall at its axis of movement, a ring of insulating material surrounding said block supported for movement with said chamber, a metallic contact sleeve secured to the outer end of said block, an annular tongue and groove connection between said sleeve and ring, whereby said block and electrode are centered at the axis of rotation of the chamber, and an electrical connection to said sleeve.

4. In an electric furnace, an electrode, terminal blocks electrically associated with the ends of the electrode, electrical conductors arranged for connection to said blocks, and a plurality of concentric ring shaped spacer contacts disposed between the conductor and blocks at each end of the electrode, said contacts being split to prevent circulating currents therein and composed of current carrying materials having diflerent electrical and thermal conductivities.

5. In an electric furnace, an electrode, terminal blocks electrically associated with the ends of the electrode, electrical conductors for connection to said blocks, a plurality of concentric ring shaped spacer contacts disposed between the conductors and blocks at each end'of the electrode, said contacts being split to prevent circulating current therein and composed of current carrying materials having different electrical and thermal conductivities, and shield plates of thermal insulating material at the ends of said spacer contacts.

6. In an electric furnace, an electrode terminal having low resistance and low heat conductivity, which comprises a graphite electrode terminal block, a metallic sleeve containing nickel and chromium substantially in the proportions of eighty per cent. to twenty per cent. respectively, said sleeve being disposed around an end of said block, a sleeve of electrical conducting material concentrically disposed around the first metallic sleeve, a carbon spacer between said sleeves, and a plate of thermal insulating material extending over the outer ends of the block, nickel-chromium sleeve and carbon spacer.

7. In an electric furnace, a melting chamber having end walls and supported for rotative movement, electrode terminal blocks respectively projecting through said end walls, a concentric tongue member carried by each terminal block, a concentric groove member carried by each end of said chamber, the tongue and groove members at each end of the chamber being operatively associated to enable relative movement therebetween and center the associated terminal blocks at the axis of rotation of the chamber, and an electrode within said chamber having its ends respectively secured to the terminal blocks, whereby the tongue and groove members are maintained in interfitting relationship.

8. In an electric furnace, an electrode terminal construction of low electrical resistance and low heat conductivity, which comprises, a terminal blockof graphitic material adapted to make'electrical contact with the furnace electrode," a metallic ring structure surrounding an end of said block and making electrical contact therewith, a second metallic ringstructure concentrically disposed around the first ring structure, the-metals of said ring structures nal construction of low electrical resistance and low heat conductivity, which comprises, a terminal block of graphitic material adapted to make electrical contact with the furnace electrode, a. metallic ring structure surrounding one end of said block and making electrical contact therewith, a second metallic ring structure concentrically disposed around the first ring structure, the

metals of said ring structures being of diiTerent composition and having difi'erent electrical and heat conductivity characteristics, and a carbon spacer between the ring structures.

CLAUDE M. WEINHE'IMER. PAUL E. CARDELL. 

